If your modulation is FM, the receiver audio is going to remain the
same. And yes, the S meter will jump due to excess noise output of the
preamplifier. The Excess gain will reduce the dynamic range of your
receiver system and cause intermodulation (IM).

1. Connect a SINAD meter to your receiver .
2. Connect the Cushman directly to the receiver, adjust for +/- 3.0 KHz
deviation with 1000 Hz tone.
3. Adjust Cushman generator for 12 dB SINAD
4. Note the generator output in dBm
5. Install preamplifier between Cushman and receiver
6. Adjust generator again for 12 dB SINAD
7. Note the generator output in dBm (should be more negative value than
step 4)
8. The difference in dBm from steps 4 and 7 relate to the improvment (if
any) in noise figure between the receiver alone and the receiver with
the preamplifier.

9. Install a variable attenuator between output of preamplifier and
input of receiver. Set to 0 dB attenuation.
10. Adjust generator for 12 dB SINAD
11. Adjust variable attenuator from step 9 until the 12 dB SINAD
degrades slighly (numerically 12 dB).
12. Readjust attenuator 1 dB per step until 12 dB SINAD is restored.
13. You have optimized your receiver and preamplifier for best
intermodulation (IM) performance.
14. Install preamplifier close to antenna with a feedline, replace
variable attenuator with fixed value equal to the value from step 12
minu the measured loss of the feedline between preamplifier and receiver.

15. If Cushman doesn't display dBM, use microvolts, fewer microvolts for
12 dB SINAD is the goal!

16. The above assumed an unmodified, properly operating preamplifier.
Your preamplifier must have some sort of preselector filter between the
antenna and it's input. Removing the stock filter exposes the
preamplifier to overload from out of band signals, may affect the
impedance (stability) of the preamplifier. Try operating it in its stock
configuration and frequency prior to modifieing it. Then replace the
stock filter with a properly tuned filter for the frequency you desire.
Yes, you might have to retune the preamp if the frequency change is too
far. Use the SINAD method and a low level signal to tweak the
preamplifier sensitivity. Make sure your generator and cables are 50
ohms. A 10 dB 50 ohm pad between generator output and the system, may
improve your results if the generator does not provide a 50 ohm source.




wrote:

Im guessing at the noise figure, im running a low level 1mv fr0m a
cushman service monitor to the preamp input. Im using a uniden 898t
scanner as a receiver because it has a digital S meter. When i turn on
the preamp jumps up 3-4 bars
but the modulation tone stays the same volume level. By this im
guessing the noise level has increased, the preamp had a ceramic
filter with the 50ohm input going directly into it and its output thur
cap to the 1st amp, the seller on ebay said just remove the filter and
move the input to the filters output trace, thats what i done but
results are poor even thou this is a commercial preamp pulled from
analog cell site. It looks to be a very well made preamp so it should
work if i figure out the
input match?




--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P